This invention relates to semiconductor lasers and more specifically to a control system for selecting optimal operating conditions in multichannel laser sources.
An ever increasing need for information transmission has led to an increase in optical transmission systems. Optical transmission sources, such as lasers are becoming more and more complex. Recent advances in wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) require accurate wavelength lasers that are capable to function in multi-channel transmission arrangements.
In a dense wavelength division multiplexed optical network, multiple channels of different frequency or wavelength are coupled to a single optical fiber so as to increase the information capacity carried through the system. The channel density of such networks has increased dramatically resulting in narrower frequency spacing between channels. The frequency or channel spacing is narrowing from a typical 100 GHz range to about 50 GHz and even 25 GHz range.
One family of semiconductor laser that is used in such multi-channel transmission arrangements is known as distributed Bragg reflector laser (DBR), which is capable of generating a plurality of lights with desired wavelengths with a narrow channel spacing. A DBR laser has a laser cavity situated between two reflective surfaces. A tuning current signal is provided to one or both reflective surfaces to change the index of refraction, which in turn varies the wavelength of the laser light generated by the DBR.
In a multichannel laser system such as one that employs a DBR laser, it is important to control laser side mode suppression ratio (SMSR) at a desired level, so as to prevent cross-talk and transmission error. Typically, for each channel of the DBR laser a ratio of xe2x88x9230 to xe2x88x9240 dB suppression is necessary for ensuring proper transmission. However, within each desired channel, there are tuning current signal values that can cause the suppression ratio to decrease dramatically. As such, prior art systems do not typically use DBR lasers in order to vary laser wavelength. Rather, a plurality of distributed feedback, DFB, lasers are employed, with each one tuned to a desired wavelength.
Some prior art systems that have attempted to control laser side mode suppression ratio (SMSR) employ analog techniques based on generating a second harmonic tone from an analog tone that is superimposed onto the laser drive current. This technique may cause transmission errors in certain applications, and therefore is not desirable.
Thus, there is a need for a control system for a multichannel laser source, such as a DBR laser, that maintains a desired operating point within each wavelength channel.
In accordance with one embodiment of the invention, a control system in a multichannel laser controls the tuning current provided to the laser source by measuring the slope values of laser power as a function of tuning current. The laser power may be measured at its front face, where the generated light is directed to an optical fiber, or in the alternative, may be preferably measured at its back face, where portion of the emitted light can be measured. The tuning current is then varied until the system measures a desired slope value. The desired slope value relates to an area of the curve, representing the laser power, preferably backface power, versus tuning current, that assures an acceptable laser side mode suppression ratio, or other optimal operating parameters. In accordance with another embodiment of the invention, the control system also ensures that the laser source is operating so as to provide a desired frequency.
In accordance with another embodiment of the invention, a microprocessor-based control system is employed. A look-up table stores all desired slope values for each operating channel. The desired slope values are measured in advance, based on the characteristics of the laser source. Thereafter, an optical discriminator is employed to provide the laser power source value and the frequency of the light source to the microprocessor. The microprocessor provides an increment in tuning current over several iterations, so that the average slope values of the laser backface power source is calculated. The system then determines which value of tuning current corresponds to a slope closest to the look-up table slope value for a specified channel, and slowly steps the tuning current to the desired value.
The control system in accordance with the present invention allows multichannel optical laser sources, such as DBR lasers to operate flexibly over a wide range of available channels, while avoiding operating points that experience low side mode suppression ratio or other undesired operating parameters.